Recognition sites (receptors) present in the central nervous system are physiologically modulated by endogenous Transmitters and modulators. Furthermore, it is believed that many psychotropic drugs exert their therapeutic actions by affecting these receptors. Several receptor systems are currently under studys including receptors for benzodiazepines, tricyclic antidepressants, and adenosine. The role and regulation of these systems are investigated by: 1) Biochemical studies of recognition sites. It is believed these sites may be physiologically involved in anxiety, convulsions, muscle relaxation, and sleep (benzodiazepine receptors), depression (tricyclic antidepressant receptors), and sleep/wakefulness (adenosine receptors). 2) Characterization of endogenous modulators (ligands) of these receptor sites. 3) Development of appropriate in vitro models for these receptors. 4) Structure-activity studies of chemically modified substances which are related to putative endogenous ligands or modulators of these receptor sites. 5) Behavioral and pharmacologic manipulation of experimental animals to perturb these systems, combined with an examination of alterations in the character of these receptor sites.